1. Field of the Invention
The present invention relates to a valve for opening and closing a flow passage of a fluid.
2. Description of the Related Art
The following valve is employed as a valve for controlling a flow rate of a fluid.
The valve concerned is constructed such that a flow passage defined within a valve casing having fluid inlet and outlet portions is partitioned by a valve seat, with a valve hole of the valve seat being opened and closed by a valve element supported to a valve shaft. The valve shaft is driven back and forth in its axial direction by opening and closing drive means, closing the valve hole with the valve element supported thereto as it moves forward and opening the valve hole as it moves backward.
Conventionally, as a valve of this type, there is known one which uses a pulse motor as the above-mentioned drive means and in which the rotary force of the pulse motor is converted into linear motion to move the valve shaft forward and backward, thus opening and closing the valve hole (see, for example, JP 2608579).
With the valve described above, however, both the opening and closing of the valve element must be effected by driving the pulse motor. Therefore, when, for instance, a need arises to effect emergency valve closure, it is difficult to close the valve instantaneously. Further, when power outage occurs while the valve is open, the valve cannot be closed, which may cause a defect in equipment using a fluid.